1. Field of the Invention
The present invention relates to an encoder and, for example, an optical linear encoder.
2. Description of Related Art
Currently, as a type of apparatus for measuring a displacement, an optical linear encoder has been known. The optical linear encoder includes a scale and a detection head that moves along the scale. The scale is provided with a reference pattern for detecting a reference position and a main pattern for detecting a relative displacement between the scale and the detection head. The optical linear encoder detects the reference position by detecting a reference signal having a width equivalent to the pitch of the main pattern while using the reference pattern formed on the scale as a reference. An absolute positional relation between the scale and the detection head can be detected by taking the above-described displacement into account while using the reference position as the reference.
As a method for determining a reference position, a method in which the phase of a main signal indicating a detection result of a main pattern is synchronized with that of a reference signal has been known. For example, a technique for including a reference pattern in a main pattern in order to synchronize the phase of the reference signal with that of the main signal has been proposed (Japanese Unexamined Patent Application Publication No. H10-160442)
Further, a method in which a reference pattern and a main pattern are disposed in mutually different tracks and the phase of a reference signal is synchronized with that of a main signal in order to fine-adjust (i.e., fine-correct) rotational mechanical misalignment around a light incident direction has been disclosed (Published Japanese Translation of PCT International Publication for Patent Application, No. 2008-503745)